Nexus 6P - Rooted...Need original build number????????

I recently rooted my Nexus 6P. Ran into some issue's while rooting, soft bricked my phone, but was finally able to add Pure Nexus ROM and everything is running smoothly now. (Like an ***** i did not backup initially....good thinking i know)

When the time comes for me to flash stock ROM back onto the Nexus 6P, I will need my build number. But that can no longer be found in settings. Any ideas where/how i can find this?